Question

  <script language="javascript">
    // random strings
    var strings = ["How is it even possible?", "Have you ever heard about this?"];
    var WriteLine = function(s)
    {
      var end=s.length, sc=s.split("");
      for(counter = 0;counter<end;counter++)
      {
        setTimeout(function(){document.write(sc[counter]);},20);
        //                                   ^^^^^^^^^^^
      }
    }
    strings.forEach(WriteLine);
  </script>

它告诉我：

undefinedundefinedundefinedundefinedundefinedundefinedundefined

问题是由于sc函数，变量setTimeout超出了范围，我已经尝试使用this

Answer 1

您的问题的答案是使用Closures，您必须“保存”上下文。您正在使用一个setTimeout，它会在一段时间后执行您的函数，并且您的for循环可以说比20 ms更快，所以当您的setTimeout执行时，您的函数计数器已经是s.length，这就是您未定义的原因

// random strings
var strings = ["How is it even possible?", "Have you ever heard about this?"];
var WriteLine = function(s)
{
  var end=s.length, 
      sc=s.split("");
  for(counter = 0;counter<end;counter++)
  {
    setTimeout((function(c){
      document.write(sc[c]);
    })(counter),20);
    //                                   
  }
}
strings.forEach(WriteLine);

setTimeout变量超出范围

1 个答案: